1. Field of the Invention
The present invention is directed to silicate based glass compositions with boron compounds included in the composition as performance modifiers and the methods of making the composition and glass articles formed therefrom. More particularly, the present invention is directed toward the use of metal borides in silicate based glasses for the improvement of spectral properties such as the absorption of infrared energy.
2. Summary of Related Art
Performance modifiers and colorants are often added to base glass compositions to impart specific color and/or energy transmittance properties in the finished glass. The absorption of energy at specific wavelengths is often desirable to enable various uses for the glass compositions. Additionally, certain colors are preferred for various glasses for aesthetic reasons. There are limitations within specific glass compositions that must be balanced or optimized when attempting to achieve desired color and energy transmittance properties. For example, certain ingredients may improve the absorption of near infrared energy while imparting an undesirable color or reducing the light transmittance. Thus, the optimization of a specific color or energy transmittance property often negatively impacts other desirable transmittance properties.
The present invention utilizes metal borides in silicate based glass compositions to improve energy transmittance properties of the finished glass without adversely affecting the color of the glass. The use of a metal boride results in the unexpected improvement of energy transmittance properties when added to conventional silicate based glass compositions. Different metal borides will impact the silicate based glass in different manners. Thus, the selection of a specific metal boride is dependant upon the silicate based glass composition, the additional colorants in the composition, and the desired color or energy transmittance properties. For example, the inclusion of a metal boride in a soda-lime-silica glass, containing an iron colorant, exhibits a reduction in the transmittance of near infrared energy over similar non-boride containing compositions, without adversely affecting the visible light transmittance.
Silicate based glass batch compositions may generally include oxygen-containing boron compounds, such as boron trioxide(B.sub.2 O.sub.3), for the control of physical properties (eg. thermal expansion, viscosity, density and the like) of the glass. The oxygen-containing boron compounds do not have a significant effect upon the color or the transmittance properties of the finished glass. For example, a silicate glass batch composition containing boron trioxide, but without conventional colorants, will result in a clear finished glass.
Additionally, U.S. Pat. No. 3,885,855 discloses the use of boron compounds, specifically metal borides of the group including titanium, zirconium, hafnium, vanadium, niobium, and tantalum borides, as thin coatings applied onto glass substrates. The coatings are utilized to reflect light in the near infrared region.
Infrared absorbing, or heat reducing, silicate glasses are also known within the art. In general, infrared absorbing silicate glasses involve the addition of specific colorants that impact the color and energy transmittance properties of the glass.
U.S. Pat. No. 4,792,536 discloses a process for producing an infrared energy absorbing glass, containing a low total iron concentration which is highly reduced to FeO. It is further disclosed that the infrared energy absorption can be increased by including greater amounts of total iron in the glass composition, but states that the visible light transmittance would thereby be reduced below levels considered adequate for automotive glazings. The disclosed process utilizes a two stage melting and refining operation, which provides highly reducing conditions so as to increase the amount of iron in the ferrous state for a given low total iron concentration of from 0.45% to 0.65% by weight. The patent teaches that the iron must be at least 35% reduced to FeO.
Another example of an infrared absorbing silicate glass is found in U.S. Pat. No. 5,077,133. The patent discloses a green colored infrared and ultraviolet absorbing silicate glass which includes an amount of ceric oxide, or alternatively ceric oxide and titanium dioxide, and a high concentration of moderately reduced iron. The glass composition exhibits a visible light transmittance of at least 70% and a total solar energy transmittance of less than 46%. Although the glass composition exhibits a low solar energy transmittance, it is desirable to further reduce the total solar energy transmittance, through the absorption of near infrared energy, while maintaining the high visible light transmittance.
It would be an advantage to provide a performance modifier for use in a silicate glass composition that significantly improves color and energy transmittance values without adversely impacting other transmittance properties in the finished glass.